1. Field of the Invention
This invention relates to a method and apparatus for inserting threads into tubes, and more particularly to a method and apparatus for inserting optical fibers or other threads into tubes of relatively small diameters and great lengths by making use of a stream of gases.
The threads inserted into tubes according to the method and apparatus of this invention are optical fibers and other thin and long filaments, wires and cables of soft materials that can be carried forward by a stream of gases. The threads may have a single core or multiple cores or may be made up of twisted filaments. The optical fiber comprises a fiber element consisting of an inner core and an outer cladding, plus a coating of synthetic resin, metal, ceramic and other materials. The tubes are of such metals as steel and aluminum and of such nonmetallic materials as plastic.
2. Description of the Prior Art
The following is an example in which the inserted thread is an optical fiber.
Recently optical communications cables have come into wide use. Because of their low strength and high susceptibility to various environmental attacks, however, more and more optical communications cables have come to be supplied in metal tubes and other protective conduits. In some of such covered types, clearance is left between the optical fiber and the protective conduit in order to prevent the transmission loss due to the different coefficients of thermal expansion of the conduit and optical fiber. (Hereinafter, this type will be called the optical fiber cord.)
Such optical fiber cords can be made by taking advantage of a stream of fluids. An example of this technology is disclosed in Japanese Provisional Patent Publication No. 29014 of 1982 "Method of Leading Optical Fiber into Pipe". This method passes a moving member attached to the head end of a towing thread from one end to the other of a pipe by making use of the static pressure of a fluid. Then, an optical fiber connected to the towing thread is pulled into the pipe. Thus, the optical fiber is passed through the pipe after the towing thread has been passed therethrough. As such, this conventional method is complicated, troublesome and inefficient. To avoid the risk of breaking, furthermore, the towing force must be kept below the strength of the optical fiber. Because of this limitation, the method has been inapplicable to the making of such optical fiber cords as are, for example, thinner than 2 mm in diameter and greater than 30 m in length.
Other methods utilizing the stream of fluids are disclosed in U.S. Pat. Nos. 4,332,436 and 4,691,896 (EPC Patent No. 0 108 590). In these methods, pressurized fluids are passed from one end of a pipe to the other. The stream of fluid exerts a frictional force and a static pressure on the surface of an optical fiber. The frictional force and static pressure, in combination, drive the optical fiber forward through the pipe. Therefore, these methods are free from the problems of complexity and fiber breakage which exist with the aforementioned technology of Japanese Provisional Patent Publication No. 29014 of 1982. Yet, the magnitude of applicable fiber drive force is limited because the static pressure works only on an area corresponding to the cross-sectional area of the optical fiber. The peripheral surface of the optical fiber moving forward is substantially in contact with the inner surface of the pipe throughout the entire length of the fiber. Therefore, the optical fiber receives a great frictional force from the inner surface of the pipe. This frictional force works opposite to the carrying frictional force between the fluid and optical fiber. Because of this, it has taken a long time to pass optical fibers through pipes of small diameters and great lengths. Besides, it has been impossible to pass optical fibers through pipes several hundred meters long.